TIG (tungsten inert gas) welding and GTAW (gas tungsten arc welding) are welding techniques often used to join selected tubular or non-tubular pieces of low corrosion metals such as stainless steel, chrome molybdenum, nickel, titanium, and aluminum, for example.
TIG and GTAW welding requires a weld puddle atmospheric environment which is substantially shielded from non-inert gases. The elimination of non-inert gases from the surrounding atmosphere in the vicinity of the weld puddle is described as purging. There are two regions near the weld puddle that must be purged. One region is at the top of weld puddle, and this upper region is typically visible to the welder's eye. The other region is at the backside of the weld, which is not visible to the welder's eye. The welding apparatus itself will shield the exposed weld puddle visible to the eye by venting sufficient quantities of inert gas over the topside, but the backside of the weld puddle is not purged of non-inert gases by welding equipment which is typically used, because the workpiece being welded blocks the stream of inert gas from the welding equipment from reaching the backside.
The low corrosion metals typically used in TIG and GTAW welding are highly resistant to corrosion. Nonetheless, an inadequate backside purge results in detrimental carbide inclusions which may result in weld breakdown.
Quality welds are especially important in the pharmaceutical and semiconductor industries, and in other industries which utilize stainless steel or titanium pipes and tubes for selected applications. If the welds made are defective, undesired particles may be produced within the pipes or tubes. In both the pharmaceutical and semiconductor industries, submicron particles may be produced when defective welds scale or decompose. Irregularities in the pipes permit bacteria to grow and lodge within the pipes which have been defectively welded. Further, defective welds may permit leaks of toxic and poisonous gases during semiconductor processing, including such gases as arsine, silage, hydrochloric acid vapor, boron tetrachloride, and silicon tetrachloride.
Welding tubes and other workpieces according to tungsten inert gas (TIG) and gas tungsten arc (GTAW) techniques requires surrounding the weld formed in tube pieces or other pieces with a selected inert gas. Among typical inert gases are argon, helium, argon-hydrogen, argon-helium, nitrogen-hydrogen, or nitrogen. To adequately immerse sufficient portions of the workpiece being welded with inert gas, both sides of the workpiece must be provided an atmosphere of a selected inert gas or combination of gases. As noted above, the welding apparatus itself produces a stream of inert gas for application to one side (i.e., the topside) of the workpiece. When a pair of tube sections are welded together, the welding apparatus typically applies a stream of inert gas on the outer portion of the weld puddle near where the ends of the tube sections are being welded together. Additionally, a separate inert gaseous stream is applied to the interior of the tube sections being welded together, to substantially completely purge the current innertube atmosphere prior to beginning welding operation. This creates an atmosphere of inert gas on both sides of the weld puddle formed during welding operation.
For flat (or curved) solid pieces being welded together, the welding apparatus operates on one side of the adjacently disposed complementary edges of the selected workpieces being welded together, and a suitable bladder or other enclosure is constructed, by taping, for example, a sheet of substantially gaseously impermeable material onto the far side or underside of the pieces being welded. The bladder used has ingress and egress ports through which an inert purging gas will be driven. To ensure a quality weld, the purged conditions ideally exist from the beginning of welding operation and must persist during the entire welding operation and for a significant period thereafter, during which time the completed weld cools with tubes or pipes to be welded, inert gas is applied to the interior of pipe or tube sections to be welded together by connecting the end of one of the tubes or pipes to a pressurized source of inert gas.
Inert gas sources used when welding tubular, flat or curved pieces together, may have an initial charge pressure of 2000-2500 PSI. This pressure level is used to produce an inert gas flow level of approximately 25 CFH (cubic feet per hour). This flow level setting is equivalent to less than 1 PSI. (An example of less than one PSI pressure would be a person softly blowing from his or her lips.) The inert gasflow produced purges the interior of both sections of tube being welded together.
Prior to beginning purging operation in preparation for welding together two tube sections, the sections of tube will first be tack welded together, so that there will be no more than minimal leakage of inert gas through the abutting edges of tube material being welded together. It is not atypical for a volume of purging gas to be employed, which is ten times the physical volume of the tubes or the backside shielding enclosure being purged. To reduce purge gas leakage incident to welding flat or curved pieces, a shielding bladder may be taped over the backsides of the selected pieces, and a sufficient volume of inert purging gas is applied to ensure complete purging operation. A successful purge reduces the presence of oxygen to no more than about 25 parts per million at the weld site.
Although the art of TIG and GTAW welding has been known for years, an acceptably economical method for providing a complete inert gas purge for the entire weld puddle has not been developed. Current methods include (1) providing a mathematically precalculated volume of inert gas at a sufficient purge pressure a period of time presummed adequate to accomplish purge operation; (2) applying the purge gas and then checking the outlet of the tube or bladder for non-inert oxygen concentrations by lighting a match to see if the purge flow snufs it out; insuring observation of a lit match at the purge vent location and watching for the match to be extinguished; and (3) using an oxygen analyzer to assess the level of inertness of the weld puddle gaseous environment.
However, use of lighted matches to determine the presence of certain non-inert gases is generally undesirable and unreliable. Use of a formula method to mathematically project the establishment of a substantially completely inert atmosphere is generally uneconomical and unduly slow. In particular, the formula method cannot account for openings that are not sealed and cannot account for poor quality inert gas as bottle nears being empty, nor for poor quality gas from suppliers.
Better approaches to ensuring a reliable purge atmosphere on both sides of the weld position are clearly desired. Quality assurance and quality control have become critical factors in the manufacturing industry. More and more, nondestructive testing such as the borescope (a fiber optic inspection tool) are being used to visually inspect welds from the backside. These techniques are increasingly pervasive in the food, pharmaceutical, cosmetic, and chemical industries.